The present invention relates to a method of storing a gas, in particular to a method of storing a gas that has a freezing point that is substantially below 0xc2x0 C.
The term xe2x80x9cgasxe2x80x9d as used in this specification refers to a substance that is gaseous under conditions of room temperature and atmospheric pressure.
There is currently considerable interest in replacing petrol (known as xe2x80x9cgasolinexe2x80x9d in some countries) and related petrochemical fuels with fuels that produce less pollution. Substances that are being considered for use as an alternative fuel for, for example, vehicle propulsion include methanol and hydrogen. This worldwide research activity in alternative fuels is generally known as xe2x80x9cthe hydrogen economyxe2x80x9d.
One factor that is important in the commercial acceptance of an alternative fuel is the ease of storing the fuel. Petrol is a liquid at normal ambient temperatures, and so is easy to store and transport. Of the two alternative fuels mentioned above, methanol is a liquid at room temperature (melting point xe2x88x9298xc2x0 C., boiling point 65xc2x0 C.), but hydrogen is a gas at room temperature (melting point xe2x88x92259xc2x0 C., boiling point xe2x88x92252xc2x0 C.). In order for hydrogen to be widely used as a fuel, one problem that must be overcome is to find a convenient way of storing hydrogen. Various methods of storing hydrogen are currently being explored, such as storing hydrogen as a liquid, as a high-pressure gas, as a metal hydride, and storing by cryosorption. The present invention relates to a cryosorption method of storing hydrogen or other gases.
It is well known that hydrogen, and other can be physisorbed into be microscopic pores of materials such as zeolites (alumino-silicates) and xe2x80x9cactivatedxe2x80x9d carbons. Materials suitable for physisorbing gases are commercially available The physisorption cm be achieved at room temperature by high pressure absorption, but it can more effectively be carried Out by physisorption at cryogenic temperatures.
The use of carbon to store hydrogen cryogenically in this manner is disclosed in, for example, U.S. Pat. No. 4,716,736, by Amnkwah et in xe2x80x9cInt J of Hydrogen Energyxe2x80x9d Vol 14, pp 437 (1989), and by Carpetis et al in xe2x80x9cInt J of Hydrogen Energyxe2x80x9d Vol 5, pp 539 (1980). The temperature at which storage is found to be cable is variously reported to be between 4xc2x0 K and 15xc2x0 K (0xc2x0 C.=273xc2x0 K), and the stored hydrogen is released for use by a controlled increase in the temperature of the absorbent material.
It will be appreciated that this method of storing hydrogen that the absorbent material is kept at a temperature of 15xc2x0 K (xe2x88x92123xc2x0 C.) or below, and it is expensive and inconvenient to do this. In order to make a cryosorption method of storing gas more economically attractive it is desirable to raise the temperature at which the hydrogen can be stored.
In the early development of vacuum pumps it was discovered that mates such as the abovementioned zeolites and activated carbon could aid the evacuation of air from a vessel by the phenomenon of xe2x80x9ccryopumpingxe2x80x9d. Pumps that operate on the basis of this phenomenon are now commercially available, and are known as xe2x80x9ccryopumpsxe2x80x9d. It was noted during these early developments that when a vapour such as water is condensed it is possible that another gas could be within the condensed vapour. This phenomenon is known as cryotrapping. For example, the cryotrapping of nitrogen gas and argon gas in condensed water vapour is reported by Schmidlin in xe2x80x9cTransactions of 9th American Vacuum Symposiumxe2x80x9d pp197 (1962), MacMillan N.Y. Hengevoss et al have reported, in xe2x80x9cTransactions of 10th American Vacuum Symposiumxe2x80x9d pp101 (1963) MacMillan N.Y., the cryotrapping of argon in condensed water vapour. The present inventor has also demonstrated cryotrapping of hydrogen and argon simultaneously when water vapour is condensed as ice under vacuum conditions, It is possible to show that a gas has been cryotrapped in, for example, condensed water vapour by detecting the subsequent release of the entrapped gas upon the heating of the condensed water vapour to a temperature, typically 160-200xc2x0 K, at which the water remain frozen. This can be done, for example, using a mass spectrometer. The phenomenon of xe2x80x9ccryotrappingxe2x80x9d is now well known in the field of vacuum technology.
Cryotrapping can be carried out using an absorbent storage medium, such as the above-mentioned zeolites or activated carbon. (The use of an absorbent medium is not essential, however, and cryotrapping may be performed by entrapping a gas in a condensing vapour in the absence of an absorbent storage medium.) Where a gas with a low boiling point, for example such as hydrogen or argon, is cryotrapped into an absorbent storage medium by condensing water vapour, the hydrogen or argon is trapped in the condensed water vapour during the process of condensing the water vapour. Water has a lower vapour pressure than either hydrogen or argon, so that the pores of the storage medium will be preferentially filled with water-ice rather than with hydrogen or argon. The capacity of the storage medium to store hydrogen or argon is thus severely restricted This method is therefore unsuitable for the large-scale storage of gases such as hydrogen.
WO 00/01980 describes a method of storing a gas. The gas to be stored is liquefied, and is then absorbed onto an absorbent medium such as activated carbon. The absorbent medium is then encapsulated, for example using water-ice as an encapsulant.
The method of WO 00/01980 is unsuitable for commercial storage of gases having a low boiling (liquefaction) temperature, since the method requires that the gas to be stored is liquefied. For example, storing hydrogen gas by the method of WO 00/01980 would necessitate cooling the hydrogen gas to xe2x88x92252xc2x0C. in order to liquefy the hydrogen gas, and this would be very expensive In practice, cooling hydrogen gas to xe2x88x92252xc2x0 C. would entail the use of liquid helium (boiling point: xe2x88x92269xc2x0 C.) as a refrigerant. The need to use liquid helium would require special cryogenic apparats, and this would be expensive to provide and maintain. Liquid helium itself is also expensive.
It would therefore be very cumbersome and expensive to store hydrogen gas using the method of WO 00/1980, so that WO 00/1980 does not provide a commercially acceptable method of storing hydrogen gas.
A first aspect of the present invention provides a method of storing gas comprising the steps of: absorbing gas onto/into an absorbent medium; and encapsulating the gas by freezing an encapsulant material onto the absorbent medium.
In this aspect of the invention gas is absorbed onto/into the absorbent medium in the absence of the encapsulant material, so that the encapsulant material does not compete with the gas to fill the pores of the absorbent storage medium. In consequence the pores of the absorbent medium are filled with the gas to be stored not with the encapsulant material, and the capacity of the absorbent medium to store the gas is increased significantly, probably by orders of magnitude, compared to a conventional co-condensation/cryotrapping method.
In the invention, the gas to be stored is absorbed onto/into the absorbent medium in the gaseous state, so that it is not necessary to liquefy the gas to be stored. Encapsulating the gas increases the maximum temperature at which the gas can be kept without release of the gas. The invention thus provides an improved method of storing a gas, in particular of storing a gas having a low boiling point.
A second aspect of the present invention provides a method of storing and releasing gas comprising the steps of: storing gas by a method as defined above; and increasing the temperature of the stored gas to a temperature at which stored gas is released through the frozen encapsulant material.
A third aspect of the present invention provides a stored gas comprising a gas-charged absorbent medium; and an encapsulant layer encapsulating the absorbent medium, the encapsulant layer having been formed by freezing an encapsulant material.
Preferred features of the present invention are set out in the dependent claims.